1. Field of the Invention
The present invention relates to the transfer of thermal energy to or from a fluid and particularly to the chilling of potable water. More specifically, this invention is directed to apparatus for exchanging heat between a pair of fluids, one of which is disposed in or being delivered to a reservoir, and especially to direct expansion-type evaporators for use in the chilling of liquids. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to employment in drinking water fountains for the chilling of potable water. The conventional manner of chilling water for such drinking fountain use utilizes mechanical refrigeration to remove heat from potable water contained in a cylindrical reservoir. Thus, heat is removed from the water by an evaporator, the heat subsequently being discharged to the atmosphere via a condenser. The evaporator in such conventional coolers will typically comprise metal tubing wrapped in a spiral around the outside of the water reservoir. As refrigerant evaporates inside the tubing, a low temperature is developed and, accordingly, thermal energy will migrate from the higher temperature potable water through the reservoir wall and the tubing wall, and will be transferred into the refrigerant. As this heat migration occurs, the temperature of the potable water in the reservoir will be reduced.
The tubing comprising the evaporator has, in the prior art, typically been wrapped around the outside of the reservoir, rather than being submersed within the potable water, because safety codes require an atmospherically vented, double wall separation between the refrigerant, which is toxic, and the potable water. With such double wall separation, external leakage of either the refrigerant or the potable water would be signified by the flow of either fluid into the atmospherically vented space between the walls. It therefore follows that a commercially available atmospherically vented, double-walled tube could be immersed in a reservoir of potable water and refrigerant circulated through the inner conduit of the double-walled tube. In such case, if a leak in the inner refrigerant conducting conduit were to develop, the refrigerant would vent to the atmosphere through the space defined about the inner conduit by a coaxial outer conduit and would not contaminate the water.
In the above-described conventional water chilling evaporator, in the drinking water fountain environment, a reserve volume or reserve capacity of chilled water is contained by the reservoir. This reserve capacity is required to accommodate instantaneous demand for chilled water when the drinking fountain is used. When the reserve capacity of chilled water is depleted, i.e., when the water in the reservoir becomes warm due to the warm temperature of the inflowing water which replaces outflowing chilled water, the refrigeration system must be capable of chilling the water in the reservoir to a given temperature within a given recovery time. The physical size of conventional water chilling evaporators is proportional to the reserve capacity of the apparatus and inversely proportional to the recovery time. While it is beneficial to maximize reserve capacity and minimize the recovery time, the physical size of such conventional evaporators is often larger than deemed acceptable or desirable.
It is also to be observed that the above-described prior art method and apparatus of water chilling is inefficient because of a heat transfer resistance, known in the art as contact resistance, which exists between the wall of the reservoir and the evaporator tubing. Another related inefficiency of the above-described prior art is that water contained within the reservoir is for all practical purposes stagnant, i.e., water velocity across the inner wall of the reservoir is so low that the film coefficient of heat transfer is seriously impaired. As a result of both of these effects, i.e., contact resistance and impaired film coefficient, the overall heat transfer coefficient is diminished and the evaporator is typically oversized with very long lengths of coiled tubing and an oversized potable water reservoir in an attempt to achieve the desired recovery time and reserve capacity. The oversizing of the evaporator tubing and reservoir increases the size, weight and cost of the water cooler.
Theoretically, the efficiency of a direct expansion water chilling evaporator could be enhanced by immersing the tubing coil directly within the chilled water reservoir to eliminate the contact resistance. However, as noted above, conventional single wall tubing would be unacceptable because of the requirement for vented double wall separation between the refrigerant and potable water as dictated by safety considerations. As also noted above, the requirement for vented, double wall separation could theoretically be met through the use of commercially available double wall heat transfer tubing, spirally fluted double wall tubing for example, which could be coiled and immersed within a water reservoir. In such case, any leak in either the inner or outer tube wall would result in the migration of the fluid passing through the leak to the tube ends where the leak would vent to the atmosphere.
When compared to conventional tubing, a spirally fluted double wall tube has a much higher heat transfer efficiency as a result of its inherently low contact resistance between the two tube walls. The inherently low contact resistance is the result of a good mechanical bond which is promoted by a high degree of contact pressure between the two tube walls. Thus, at first glance, it might appear that acceptable water chilling in a drinking fountain could be accomplished through the use of spirally fluted, double wall tubing which is formed into a coil and immersed in the reservoir and vented. This, however, is not the case since the heat transfer from the chilled water would be limited by the fact that the water would be approximately stagnant on the outside surface of the fluted tube. Restated, because of the insignificantly low velocity of water flow over the surface of the spirally fluted outer tube, the film coefficient on the outer surface of the fluted tube would be seriously impaired, thus resulting in an unacceptably low overall heat transfer coefficient.